Fluorinated polymers are not degraded when exposed to chemical systems commonly encountered in chemical processes and therefore have been used to coat chemical processing equipment. In general fluorinated polymers are inert to strong mineral acids, inorganic bases, inorganic oxidizing agents and salt solutions. Such fluoropolymers are also inert to organic compounds such as organic acids, anhydrides, aromatics, aliphatic hydrocarbons, alcohols, aldehydes, ketones, esters, chlorocarbons, fluorocarbons, and mixtures of these organic compounds.
However, in order to be effective against corrosion such fluoropolymer coatings need to exhibit (1) good wet adhesion and (2) low water (solvent)/oxygen permeability. By good wet adhesion is meant the adhesion of the polymer film to a substrate in the presence of water (solvent). By low water (solvent) and oxygen permeability is meant the ability to retard water (solvent) and oxygen permeation through the polymer film to the underlying substrate, usually metal. Ways to improve the corrosion resistance of fluoropolymer coatings have been desired.
Films of various fluoropolymers which can have various fillers including mica, are used for cladding metal substrates in Japanese Kokai 4-229246, Sahara et al. (1992). There is no focus in this Kokai on minimizing permeation or on using mica in fluoropolymers such as PFA, or on what proportions would be needed or on improving the wettability of the fluoropolymer by adding a second filler material.
Mica coated with oxides to produce sparkling optical effects is the subject of U.S. Pat. No. 3,087,829, Linton (1963).
Mica is used in fluoropolymer coatings for cookware to minimize stain formation, as in U.S. Pat. No. 4,353,950, Vassiliou (1982).
Metal laminates which have been precoated with powder base paints having various additives and fillers such as mica have been disclosed in U.S. Pat. No. 5,204,168. There is no recognition of the combination of a second filler with the mica to improve wettability of a fluoropolymer coating.
The need remains for a composition suitable for coating processing equipment which meets the chemical process industry requirements for good corrosion resistance, high temperature resistance, and good chemical inertness.